This invention relates to an industrial process for preparing organohalosilanes.
With respect to the synthesis of alkylhalosilanes, E. Rochow first disclosed in U.S. Pat. No. 2,380,995 direct synthesis reaction between metallic silicon and alkyl halide in the presence of a copper catalyst. Since then, there have been reported a number of research works relating to various co-catalysts used together with copper catalysts, reactors, additives used during reaction, and the like.
In the industrial synthesis of organohalosilanes, the selectivity of diorganodihalosilane which is most widely used in silicone resins, the formation rate of silanes, and the percent conversion of metallic silicon into useful silane are crucial. The selectivity of diorganodihalosilane is evaluated in terms of a weight or molar ratio of dialkyldihalosilane to the silanes produced and a T/D ratio. Organohalosilane products contain diorganodihalosilane (D), triorganohalosilane (M), organotrihalosilane (T), etc. as well as other by-products such as organohydrodihalosilane (H) and organohalodisilane. In particular, disilanes are known as a high-boiling fraction among manufacturers who make silicones from organohalosilanes obtained by the direct process, because few processes are available for the effective utilization of disilanes, and most disilanes are discarded. The T/D ratio is a compositional ratio of organotrihalosilane to diorganodihalosilane in the entire organohalosilanes produced, with a lower T/D ratio being preferred.
The formation rate of organohalosilane is represented by a space time yield (STY) which is the weight of crude organohalosilane produced per unit time relative to the weight of metallic silicon held in the reactor. In order to improve the content of diorganodihalosilane produced, reduce the T/D ratio or increase the STY, various research works have been made with a focus on the catalyst and co-catalyst. Also, an attempt was made to add an inert solid to the reactor for improving the results of organohalosilane synthesis.
JP-A 61-112085 discloses to add fumed silica to a fluidized bed reactor for the purpose of reducing agglomeration of cuprous chloride within the reactor.
JP-B 4-59318 discloses, for the production of halogen-bearing silanes, to add inert solid particles having a particle size distribution in the range of 20 to 450 xcexcm to a fluidized bed reactor for the purpose of facilitating the temperature control of the reaction zone within the reactor.
German Patent No. 19919337C1 discloses to use metallic silicon, copper catalyst, zinc co-catalyst and fumed silica for the purpose of reducing the amount of the catalyst used to produce methylchlorosilane.
As discussed above, it is economically advantageous for the industrial production of organohalosilanes to improve the proportion of diorganodihalosilane produced and increase the formation rate of organohalosilanes. However, these two targets are in a tradeoff relationship. An attempt to improve either one of the targets results in a failure to improve the other target. It has been an outstanding issue for engineers to find a solution to overcome the tradeoff phenomenon.
An object of the invention is to provide a process for producing organohalosilanes which process can increase the formation rate of organohalosilanes without reducing the proportion of diorganodihalosilane produced.
The inventors have found that by mixing particles of a catalyst and/or a co-catalyst with finely divided silica, mechanically applying shear forces to the mixture for mutually rubbing the particles, thereby producing the catalyst and/or co-catalyst having finely divided silica attached to surfaces thereof, and using the resulting catalyst and/or co-catalyst in a contact mass, quite unexpectedly, the formation rate of organohalosilanes can be improved without reducing the proportion of diorganodihalosilane produced.
After catalyst particles and/or co-catalyst particles and finely divided silica are rubbed under the mechanical application of shear forces, the catalyst and/or co-catalyst is used for the synthesis of organohalosilanes. Quite unexpectedly, unlike a simple mixture, the mechanically rubbed mixture is effective for substantially improving the reactivity of organohalosilane synthesis.
Although the mechanism by which reactivity is improved is not well understood, it is presumed that when finely divided silica is rubbed against surfaces of catalyst or co-catalyst particles under shear forces, fusion between the catalyst or co-catalyst surfaces and finely divided silica surfaces, which is referred to as xe2x80x9cmechanical alloying,xe2x80x9d occurs. This modifies the physical and chemical characteristics of the catalyst or co-catalyst, as a result of which the formation rate of organohalosilane is significantly increased. Presumably, changes of the shape of catalyst or co-catalyst particles, changes of the surface topography of catalyst or co-catalyst particles, and changes of other factors by the mechanical effects cooperate with changes of the catalysis by the mechanochemical effects, in a complex manner to bring about an increased formation rate of organohalosilanes. The modification of catalysis by such shear force-applying operation is a unique effect that does not occur by merely mixing the catalyst with finely divided silica, but occurs only when shear forces are imparted, as will be demonstrated by Examples to be described later.
Accordingly, the present invention provides a process for preparing oganohalosilanes comprising the steps of charging a reactor with a contact mass containing a metallic silicon powder, a copper catalyst and a co-catalyst, and introducing an organohalide-containing gas into the reactor to effect reaction to form organohalosilanes of the following general formula (1):
Rn(H)mSiX(4xe2x88x92nxe2x88x92m)xe2x80x83xe2x80x83(1)
wherein R is a monovalent hydrocarbon group of 1 to 6 carbon atoms, X is a halogen atom, n and m each are an integer of 0 to 3, and the sum of n and m is 1 to 3, the process further comprising the steps of mixing particles of the catalyst and/or co-catalyst with finely divided silica, and mechanically applying shear forces to the mixture for mutually rubbing the particles, thereby producing the catalyst and/or co-catalyst having finely divided silica attached to surfaces thereof, which is used in the contact mass.